Learning Forum

[Ramida Rungroj]

This question is not related to the ansys but I want someone to give me some ideas.

This is my structural design of the wing and the result of the Modal analysis. I'm not sure that my design is considered to be good or bad according to my result. Can someone give me some idea of what I have to do next when I get this result?

[peteroznewman]

I suggest you take the following free courses to learn what you can do with a Modal analysis result.

/courses/index.php/courses/intro-to-modal-based-methods/

/courses/index.php/courses/modal-analysis-in-ansys-mechanical/

/courses/index.php/courses/modal-analysis-of-a-wing/

/courses/index.php/courses/harmonic-response-analysis-in-ansys-mechanical/

/courses/index.php/courses/mode-superposition-transient-analysis/

/courses/index.php/courses/random-vibration-analysis/

/courses/index.php/courses/single-point-response-spectrum-analysis-using-ansys-mechanical/

[Ramida Rungroj]

Thank you so much! but can you give me an example of how can I improve my structure? I try to reduce the mass of the internal structure but it doesn't help

[peteroznewman]

How are you defining improvement?  It is possible to make changes to increase the first mode above 17 Hz.  Review these videos to learn where to increase stiffness and where to reduce mass.

https://www.youtube.com/watch?v=sFNGXjlnIFQ

https://www.youtube.com/watch?v=2xJWO3F6i7c

Another way to increase the first mode is to increase the thickness of the wing. However, the first mode is not the only metric to define the quality of the wing. When the wing is in flight, there is a pressure load on the wing surfaces that cause the wing to flex upward.  A simple way to approximate that pressure load is to take half the mass of the plane, multiply by g and divide by the area of the wing plan to compute a pressure to apply to the bottom surface of the wing. 

I expect a larger load is when the plane lands. There would be some maximum upward acceleration as the landing gear compresses to arrest the downward velocity of the plane on touchdown. Many plane designs put fuel tanks in the wings so the worst case wing load will be to land with full fuel tanks.

Good luck!

[Ramida Rungroj]

I see Thank you so much! My professor said that I have to improve the structure of the wing but my problem is I am not sure that now my structure is good or bad according to the result.

[peteroznewman]

The structure is good if it meets the requirements and bad if it doesn't.  What are the requirements?  Aerospace structures often have two requirements:  a minimum first mode frequency and a maximum mass. 

If the minimum first mode requirement is 30 Hz, then your 17 Hz structure is bad, if the minimum first mode requirement is 15 Hz, then your structure is good unless it is too heavy, then it is bad. 

An improved structure is one that is either lighter at the same first mode or has a higher first mode at the same mass or is both lighter and with a higher first mode. The videos I provided point you to where on your structure there is insufficient stiffness (strain energy density is too high) or excessive mass (strain energy density is too low).  The strain energy density of an ideal structure has a smaller range of strain energy density.

[Ramida Rungroj]

Oh, one last question, how can I set the requirement? the design criteria of the wings will include them being able to have more than 2 times the factor of safety (stress), and deformation of less than 10% of the wingspan. We have not set anything about the frequency, can you give me an example of how to set it?

[peteroznewman]

There may be no need for a requirement on the first mode frequency on this wing. I don't know of one. In other structures, one reason to have a first mode requirement is because there is a vibration load that acts on the structure. For example, a pump runs at a certian frequency and has an imbalance. You don't want the support structure to have a matching modal frequency because the structure will be induced to vibrate at that frequency which can lead to fatigue failure. In that example, the structure may have a modal requirement called a "keep-out zone" that requires no modal frequencies between two values that bracket the known vibration frequency of the pump.

[Mare Levin]

Hi, could you please help me, i would be very grateful, i created a model in APDL and use it as an external model in Workbench for static analysis, i want to use ACP (Post) but i cant drag the Solution=>Static Structural to Results=>ACP Post